Why the Story of How the Moon Formed is Changing

The moon has been our constant companion for billions of years. But where did it come from? Since the 1970s, the popular theory centered around a Mars-sized body grazing the early Earth. This theory helps explain several things about the moon including its large size and the rotation rates between the Earth and the moon.

Why the different results? 40 years might not seem like a long when talking about the moon and Earth, but it’s a technological eternity for us. Instruments and analysis have seen huge leaps since the 1970s.

Last year, Kun Wang (a geochemist at Washington University in St. Louis) and Stein Jacobsen (a professor of geochemistry at Harvard University) developed a more sophisticated technique for analyzing isotopes in moon rocks gathered during the Apollo missions.

While the 1970s theory explains the physical characteristics of the Earth-moon system, it came up short when explaining their geochemistry according to the researchers.

In 2015, a new theory was proposed that said the moon formed after an extremely violent collision. Another body slammed into Earth causing the impactor and the Earth’s mantle to vaporize and mix. It formed what the model describes as a dense melt/vapor mantle atmosphere that expanded to fill a space 500 times bigger than today’s Earth. The moon condensed from this material as it cooled.

Wang says this mixing explains the identical isotope composition of the Earth and moon seen in previous studies. Those studies were looking at the isotopes of oxygen. Wang and Jacobsen looked at potassium.

Specifically, potassium-41 (heavier) and potassium-39 (lighter). There are three stable isotopes of potassium, but only those two are seen at levels high enough to be measured with precision.

Their analysis showed the lunar rocks were enriched by about 0.4 parts per thousand in potassium-41. Incredibly high temperatures are needed to separate the potassium isotopes like this. And an incomplete condensation of the potassium from the vapor phase during the moon’s formation would do the trick says Wang.

One issue did arise from the high-impact model. Calculations show if this condensing happened in an absolute vacuum than the heavy potassium isotopes in the lunar samples should be closer to 100 parts per thousand. But if you factor in higher pressure, the 0.04 parts per thousand makes sense. Wang and Jacobsen predict the moon condensed in a pressure about 10 times the sea-level atmospheric pressure on Earth.

“Our results provide the first hand evidence that the impact really did (largely) vaporize Earth,” said Wang.

But not everyone is on board with the results from Wang and Jacobsen. “That is definitely a tall claim,” Munir Humayun, a geologist at Florida State University, told The Verge. Humayun pointed to some of the rock samples being potentially contaminated with more potassium-41.

One thing is for sure, the story of how the moon formed isn’t set in stone. Look for research into our neighbor’s origins to continue.

Thinking about OSIRIS-REx

This research got me thinking about OSIRIS-REx. In 2023, the spacecraft will deliver a sample of asteroid Bennu to Earth. NASA will study it as part of the OSIRIS-REx’s primary mission. But they will also save at least 75% of the sample for future scientists to study it.

Just like with lunar rocks, the story of Bennu and its secrets will evolve over time. What we learn in the next 10 years might change over 40 years as more sophisticated instruments are designed in the future.